Valve Timing and Variable Valve Timing

Assuming that you know the basics of 4-stroke internal combustion engines (here is my brief introduction) as well as common fuel management systems I wrote about in this post, you can jump to this next topic. By now you should be aware of the importance of intake and exhaust valves’ timing in the smooth operation of the engine. In some cases such as the nitrous one, you might even want to perform some performance tuning. In this post, I will provide some information regarding valve timing and an evolutionary technology known as variable valve timing which is very common in various modern cars.

Camshafts
One of the most important component of this blog post is known as camshaft.

As you can see, it is nothing more than a simple shaft with numerous cams attached to it. In fact, it has as many cams as the number of the valves it is designed to handle. Below you can see an animated figure, its functionality is straightforward although crucial for engine’s operation.

Depending on the design, this functionality is implemented using one or more camshafts to open and close the valves according to the engine’s four stroke cycle. Since precise timing (which is known as valve timing) is required to achieve this task, camshaft is connected to the engine’s crankshaft. The exact connection might differ from manufacturer and model but a general overview (using two camshafts) looks like this:

The connection of the crankshaft and the camshaft(s) is achieved using a so called timing belt and the ideal timing based on the crankshaft’s rotation is a result of the camshaft gears which are connected to the crankshaft using the aforementioned timing belt. In some cases, the camshaft gears are used for additional tasks such as triggering the ignition system distributor but this is out of the scope of this blog post.
Knowing these, let’s dive into a more detailed look of the cam…

Here we have three stages which are identical for both opening and closing of the valves. I will discuss them separately starting from left to right. We have:
– Ramp
The stage when the valve will start opening (for opening) or has just finished closing (for closing cycle).
– Flank
This part of the cam is the factor of how fast of slow the valve will start opening or closing depending on the side.
– Nose
The point where the valve reaches its maximum lift. This part also determines how deep will be that lift.
Although the terminology might differ, the operation remains the same. Speaking of terminology, here is a similar graph I found on the web which is also very informative.

Here you can also see some new terms which are duration and overlap. Duration is the time required (measured in degrees) from the beginning of opening/closing to reaching the maximum/minimum lift of a valve. This is one of the most important characteristics among camshaft technical specifications. Now, the overlap is an interval when both intake and exhaust valves are neither closed nor open.
An even more detailed graph is this one:

However, it doesn’t provide any additional information. I haven’t included this as the first one because it would make it look more complex than it really is.
Now, the valve timing is represented through a series of degree numbers. So, when you find a camshaft with specifications similar to these:

Before ending this section about camshafts, here are a couple of well known techniques to increase performance through camshaft.
– Lift Increase
The higher the lift the more air will be provided to the combustion chamber and more exhaust gas will be released. This translates to more power, however, this is highly affected by the duration since the increased duration (because of the cam’s shape) will decrease the performance. For that matter, this is effective but it usually provides limited results.
– Increasing Overlap
This is usually achieved by increasing the lift duration. As a result the combustion chamber will have an interval when the intake valve will provide air and the half-open exhaust valve will allow emptying the combustion chamber from any gases with higher pressure. This will lead to a more powerful ignition in the next engine’s cycle. The downside of this performance tuning is that it is only effective on high RPMs and consequently, it doesn’t provide considerable power increase in low RPMs.

Valves
Yeah, the next component will be valves and valve-train.

These are very simple parts which have a mushroom like shape. Their technical characteristics are usually their construction material as well as their diameter which is shown in the next figure.

However, in internal combustion engines, this component moves through a mechanical device known as valve-train. This is nothing more than a series of springs, push rods and rocker arms that are designed to transfer the pressure of camshaft’s cams to the valves. Here you can see a valvetrain with two camshafts (one for intake and another for exhaust valves).

The springs and rocker arms installation allows the valves to return to their original position when they don’t get pushed by the camshaft. Clearly, its name was taken from the setup which resembles a train of valves. On a close up look of a real engine’s valvetrain you can see that it is not far away from the previous figure.

So, going back to the two common performance tuning techniques used in camshafts we can now add the vavletrain factor. Performing slight modifications by replacing just the camshaft(s) is feasible. However, most higher level tuning will require changing the valves and valvetrain completely. By doing this you can achieve the end result easier since you have the ability to select the appropriate springs, valves, etc. Happily for us, many aftermarket manufacturers provide ready to use kits such as the one you see here:

This is a racing level, high tolerance kit by JUN which includes everything from titanium valves, lightweight tuned camshafts, camshaft gears, etc. and it will cost you roughly about $4,000 – $5,000.
That said, you should be wondering by now how the same setup applies to different engine configurations. This will be my next topic in this blog post.

I will start this subject we our beloved V-Twin engine. Have a look at the following V-Twin engines for a second and imagine their internal parts such as crankshaft, pistons, valves, etc.

I am fairly sure that all of you have seen those shiny chrome tubes (two on each cylinder in the above picture). These are called pushrods and they are used to transfer the pressure to the cylinders’ head where the valves reside. Here are some stripped (no cover, springs, etc.) pushrods.

These are placed inside some cool looking metal covers like these:

And end up looking like the picture you saw earlier. The covers are used only for protection and aesthetics, they don’t provide any special functionality. Here is a nice photo I found on the web which shows exactly this.

At last, before moving into the details of the various camshaft designs after this gentle introduction, here is an awesome animation of a V-Twin operation. Please, pay attention on the pushrods’ operation.

So, a similarity in all of the examples except the V-Twin I gave so far was that all of them had the camshaft(s) on top of cylinders, inside the cylinders’ head. This configuration is commonly known as over-head camshaft (OHC). This is the most widely used configuration since it provides excellent performance (force is driven directly onto the valves, no pushrods or other components) and easy maintenance (it is on top of the engine and the timing is based on timing belt). This configuration is generally separated in two different designs that I will discuss next.

Single Over-Head Camshaft (SOHC)
This works only for straight (or inline) engine configurations. As it is implied by its name, it uses a single camshaft placed in the cylinders’ head.

This is the least complex design and also a very efficient one. It usually provides great performance since it is lightweight, fast and doesn’t use any additional parts such as pushrods. Below you can see one of the most classic SOHC engines which was installed in some of the first Honda CRX.

Just by having a quick look you can say that this is a single, overhead camshaft setup.

Double Over-Head Camshaft (DOHC)
Moving to the next design, this one uses twin camshafts. From the next diagram it is very easy to guess how its name was derived.

This is a more complex design using two camshafts that are synchronized through timing belt. Each camshaft is responsible for either the intake or the exhaust valves of the engine. It’s important to note here that on a V shaped engine, having two camshafts doesn’t make it DOHC. You need two on each cylinder bank (one for intake valves and another for exhaust valves). This type of camshaft design is usually harder to repair but it provides additional performance and efficiency.

Camless
This is definitely the most advanced design which is used in very special situations. The most common setup of camless installations uses either electromagnetic or pneumatic springs to open the valves. As an example, F1 cars use the aforementioned type of pneumatic springs like the ones in this figure:

Although this type can provide great control, it is such a complex design that requires extensive monitoring and it usually needs great amount of time for diagnose and repair.

With this type we have completed our first introduction to valve timing and we can now jump to a technology known as variable valve timing. :)

Variable Valve Timing
As I have already mentioned, the camshaft’s specification can be described through a few properties which are:
– Intake/Exhaust timing
– Duration
– Overlap
– Lift
The variable valve timing (from now on I will be referring to as VVT) can change these properties during engine’s operation to achieve different goals. This can be reached using various different techniques, for example Toyota developed a technology known as VVTL-i.

This one uses an electronically controlled sliding pin that is triggered on high RPMs and thus, increases the camshaft’s lifting ability. Another very popular one is Honda VTEC.

Honda’s VTEC can change the camshaft’s profile at higher RPMs to achieve different duration and lifting. Such VVT implementations provide amazing performance gain and they are very common among different manufacturers in modern cars.

Different Variable Valve Timing Implementations
As in most technologies, manufacturers use their own names for the same thing. Here is a quick list with the most well known variable valve timing implementations.

Toyota
– VVT (Variable Valve Timing)
This was the first implementation of Toyota that was using variable timing of the intake cams.
– VVT-i (Variable Valve Timing with Intelligence)
The follow-up design of Toyota was using hydraulic (oil based) system to change the timing through camshaft gear and timing belt (mechanically) and thus achieve different overlap timing. The aim of this design was efficiency.
– Dual VVT-i (Dual Variable Valve Timing with Intelligence)
This one uses the VVT-i technology but instead of applying it just to intake valves, it also operates on the exhaust ones.
– Triple VVT-iE (Variable Valve Timing with Intelligence by Electric Motor)
Similar to the Dual VVT-i with the difference that this design uses an electric motor to change the intake camshaft’s timing. Nevertheless, the exhaust timing is still hydaulic based.
– VVTL-i (Variable Valve Timing and Lift Intelligent System)
I have discussed this earlier in this post. It changes the camshaft’s lifting ability using an electronically controlled sliding pin.
– Valvematic
The newest technology of Toyota, Valvematic uses the previous VVTL-i technology along with new electronic timing adjustment functionality.

Subaru
– AVCS (Active Valve Control System)
This is used in turbocharged engines to improve air flow. To achieve this the system uses hydraulic (oil based) support that changes the intake camshaft’s rotation. The whole implementation is controlled via vehicle’s Engine Control Unit (ECU).
– Dual AVCS (Dual Active Valve Control System)
As in the Toyota case, this does not only adjust the intake valves timing but also the exhaust ones.
– i-AVLS (Intelligent Active Valve Lift System)
A newer technology that is similar to Honda VTEC. It has two different intake lift profiles that are changed after a predefined RPM limit to increase camshaft’s lift. The whole operation is electronic using vehicle’s ECU which triggers solenoids that change the oil pressure. In addition to the variable lift system, this implementation also uses hydaulic pressure to change camshaft’s timing.

Honda
– VTEC (Variable Valve Timing and Lift Electronic Control)
I already gave a quick overview of this technology. It electronically selects between two different camshaft profiles based on the engine’s RPMs. This can change the lift, duration and valve timing.
– VTEC-E (Variable Valve Timing and Lift Electronic Control for Efficiency)
As its name implies, this is an improvement of the original VTEC to provide efficiency in the whole RPM range. This was done using an hydraulic controlled sliding pin to change the valve lift.
– 3-Stage VTEC (3-Stage Variable Valve Timing and Lift Electronic Control)
All of the previous implementations had just two camshaft profiles which were operating in low and high RPMs respectively. This one included a third one to achieve more performance in middle RPMs.
– i-VTEC (Intelligent Variable Valve Timing and Lift Electronic Control)
Apart from the 2-stage VTEC, this implementation also supports an additional lifting of the intake valves using sliding pins which are also ECU, electronically controlled.
– i-VTEC with VCM (Intelligent Variable Valve Timing and Lift Electronic Control with Variable Cylinder Management)
Similar to the previous one with the addition of VCM. The latter technology will keep some cylinders deactivated by simply keeping all of their intake and exhaust valves closed if the required power is produced without them. This was an additional fuel consumption management system.
– i-VTEC i (Variable Valve Timing and Lift Electronic Control for Injection)
Simiar to i-VTEC but designed especially for direct fuel injection engines.
– AVTEC (Advanced VTEC)
The newest Honda VVT technology which despite VTEC’s initial operation, it provides continuous variable valve timing throughout the whole RPM range using various sensors that are connected to the vehicle’s ECU.
– HYPER VTEC (Hyper Variable Valve Timing and Lift Electronic Control)
This was the first ever VVT implementation for motorcycles, it features an additional intake valve that remains closed until an RPM limit is reached.

Nissan
– N-VCT (Nissan Variable Cam Timing)
Using an ECU controlled solenoid, it alters the camshaft’s rotation and consequently, the valve timing.
– VVL (Variable Valve Lift and Timing)
Using an hydraulic, oil based system which is ECU controlled it selects between different camshaft profiles, identical to the initial VTEC design.
– CVTC (Continuous Variable Valve Timing Control)
Using hydraulic power it adjusts the camshaft’s gear and timing belt to perform the valve timing.
– VVEL (Variable Valve Event and Lift)
This is one of the most advanced we have discussed so far. The ECU uses some stepper motors to adjust the valve timing and lift throughout the operation of the engine and not after a specified threshold.

Yamaha
– VCT (Variable Cam Timing)
This was a quite innovative approach of moving the camshaft in order to have variable lift and timing.

BMW
– Valvetronic
This is a complex design where the vehicles are equipped with electronic accelerator pedal that depending on the requested load, the ECU will lift the appropriate intake and exhaust valves accordingly.
– VANOS (Variable Nockenwellensteuerung)
Most cars equipped with Valvetronic also have VANOS. This uses an advanced method of moving the camshafts so that the valve timing is adjusted based on the driving style. However, its operation is limit based (it is activated at certain RPM levels) and it affects the intake camshaft(s) only.
– Double VANOS (DoubleVariable Nockenwellensteuerung)
This one overcame the issues of the previous design meaning that it supports continuous operation in both intake and exhaust valves.

Mitsubishi
– MIVEC (Mitsubishi Innovative Valve Timing Electronic Control System)
The only VVT implementation by Mitsubishi combines various features mainly for turbocharged vehicles. Its operation in also based on cams’ profiles but they are not strictly limited to a predefined RPM limit. Depending on numerous measurements that ECU collects it might start working on different RPMs. Finally, it works on both intake and exhaust valves.

Volkswagen Group
– VVT (Variable Valve Timing)
Recently, VW included this new feature to its models. The operation is based on a hydraulic system on the timing belt that performs VVT on the intake valves.

Porsche
– VarioCam
This was a very innovative approach during the time of its development. It implements VVT by adjusting the tension of the timing belt or chain that connects the two intake and exhaust camshafts.
– VarioCam Plus
An improvement of the previous technology, this one includes electro-hydraulic lifters that can perform two-stage lifting which leads to performance similar to Honda VTEC.

Suzuki
– VVT (Variable Valve Timing)
It uses an hydraulic system that changes the camshaft’s rotation and it is ECU controlled.

Alfa Romeo
– TwinCam
Although this does not directly implements VVT, it is a slight VVT technology. Mostly, an improvement of classic DOHC. It operates using a double row timing belt that alters the valve timing between the two camshafts.

I’m fairly sure that there are many more VVT implementations that I have unintentionally omitted. In any case, I hope in the future I find some time to dive into the details of the most advanced of the aforementioned ones. Nevertheless, if you think that I missed something important here let me know to update the blog post. :)

I have almost forgot. Just to clarify, none of the above photos/pictures/figures are mine. Thanks to Google for every single one.

thanx xorl….i am doing my project on variable valve timing….this helps me a lot to understand various vvt system….my project is to develop a cheap vvt system for mid-sized car having engine from 1.2-1.6 ltr engine.
if u can help me, can u tell me various parts used in various vvt systems and which parts are expensive one….so that i can make comparison or tell me from where i could find the data…
i will wait for ur help…
thanks xorl

Thanks so much for the detailed explanation. I am a jet engine mechanic and have alway wanted to know more about recipicating engines especially V Twins. Your post really broke it down and even tought me a thing or two. Great Post